A U-shaped heat transfer tube utilized in a heat exchanger, such as a steam generator and a feed water heater, which is used in a thermal or nuclear power plant is produced by bending a small-diameter and long-length heat transfer tube with an outside diameter of 30 mm or less into a U-shape. In the U-shape heat transfer tube, an inspection is performed to detect flaws by the eddy current test from the tube inside as a pre-service inspection after the U-shaped heat transfer tube is assembled in the heat exchanger or as an in-service inspection or a periodic inspection after the U-shaped heat transfer tube is in-service for a certain period. The eddy current test from the tube inside applies strict test criteria because of the need to ensure safety of nuclear power plant facilities.
The eddy current test applying the test criteria similar to that of the pre-service inspection or periodic inspection is also required for an inspection before shipment after the heat transfer tube is produced. As a result of the eddy current test, the heat transfer tube which fails the test criteria becomes nonconforming material. Even within the test criteria, it is necessary that the result of the eddy current test be recorded in each tube while correlated with relevant positions in an axial direction of the heat transfer tube.
Usually the heat transfer tube is produced through cold working such as cold drawing and cold rolling and a heat treatment using a mother tube produced by hot extrusion. The bends in an axial direction and ovality of the tube, generated after the cold working and heat treatment, are corrected during a subsequent finishing process using a roll straightening machine. Not only many heat transfer tubes having small diameters are used in the heat exchanger, but also an installation space of the heat transfer tube becomes narrowed with miniaturizing heat exchanger. When the bend is generated in the heat transfer tube, a trouble such as interference with other parts is generated in assembling the heat transfer tube into the heat exchanger. Accordingly, it is necessary to ensure bend correction accuracy in the roll straightening machine.
Usually a cross roll type straightening machine, in which plural drum type rolls are combined, is adopted in a configuration of the roll straightening machine used in the straightening. There are many configurations in the cross type roll straightening machine according to a combination of the number of rolls, a layout (vertical and horizontal directions), and roll arrangement (opposing type and zigzag type). The roll straightening machine in which the rolls are disposed as opposed to each other is used in the step of finishing up the heat transfer tube.
FIG. 1 is a view showing an example of the roll layout of the cross roll type straightening machine. Plural pairs of straightening rolls Ra and Rb (collectively referred to as R) are provided in the roll straightening machine. The pairs of straightening rolls Ra and Rb each are vertically disposed as opposed to each other in such a state that directions of rotating axes cross in a plan view (actually, cross-wise pass each other in a spaced-apart relation in a front view). In the roll layout of FIG. 1, three pairs of straightening rolls Ra1 and Rb1, Ra2 and Rb2, and Ra3 and Rb3 are disposed on an inlet side, the center, and an outlet side respectively, the rolls of each pair being opposed to each other, and auxiliary roll Rc is provided at an exit of the outlet-side straightening rolls. Usually the roll straightening machine having such roll layout of FIG. 1 is called (2-2-2-1) type straightening machine.
An opposing rolls clearance and a cross angle can separately be adjusted in each roll of a straightening roll pair Ra1 and Rb1. A height position in a vertical direction of a first pair of straightening rolls Ra1 and Rb1 and a height position of a second pair of straightening rolls Ra2 and Rb2, adjacent to the first pair, can be also adjusted separately.
In the bend correction, a cross angle θ of the rotating axis of each straightening roll R to a tube to be corrected 1, that is, a roll angle is adjusted such that contact faces of the tube to be corrected 1 fit in contours of the straightening roll, the opposing rolls clearance between the straightening rolls Ra1 and Rb1 is set slightly smaller than an outside diameter of the tube to be corrected 1 to impart a crush, and an offset is imparted to straighten the bend and correct ovality by adjusting the crush amount of the second pair of straightening rolls Ra2 and Rb2, adjacent to the first pair.
Since high rigidity and wear-resistant properties are required for straightening rolls, the straightening roll is made of tool steel or ceramic, and the surface of the straightening roll is formed by a curved line constituting a drum shape in consideration of a contact surface with a tube to be corrected so as to enable the tube having the outside diameter within a predetermined range to be straightened. After the heat treatment, the heat transfer tube whose bends and ovality are corrected by the roll straightening machine is subjected to a process such as cutting, and the inspection before shipment is performed to the heat transfer tube by the eddy current test from the tube inside.
FIG. 2 is an example of a chart showing result of the eddy current test from inside of the heat transfer tube. As shown in FIG. 2, Signal S from Standard Flaw defined in the test criteria and signal N having a predetermined period P are shown in the chart. The signal N is called base noise, and is caused by a minute dimensional fluctuation generated in an axial direction of the heat transfer tube. It is necessary that the magnitude of the signal N be decreased as much as possible in order not to mistake the signal N for a signal caused by the detected flaw and in order to swiftly judgment whether the signal indicates the flaw to thereby improve the inspection efficiency. Hereinafter, a ratio of Signal S from Standard Flaw to Noise N is referred to as “S/N ratio”.
For example, in the case where automatic judging is made based on the signals shown in the chart during the eddy current test from tube inside, the large noise, that is, the small S/N ratio hides a signal from a small defect behind the base noise, which makes distinction between the small defect signal and the base noise harder.
Therefore, an inspector visually observes the result of the automatic sentencing in the eddy current test. When a doubtful signal is generated, the portion where the doubtful signal is generated is tested again at a lower speed to distinguish between the small defect and the base noise, thereby lowering the inspection efficiency.
As described above, the base noise is caused by the minute dimensional fluctuation generated in an axial direction of the heat transfer tube. Therefore, in order to reduce the base noise, it is necessary to suppress the dimensional fluctuation such as bends and ovality in an axial direction of the heat transfer tube, that is, to enhance the dimensional accuracy along an axial direction of the heat transfer tube.
Usually, in straightening the tube by the roll straightening machine, as shown in FIGS. 3 to 5, it is necessary that the roll angle, crush amount, and offset amount which are the setting conditions be determined to suppress the dimensional fluctuation such as the bends and the ovality in an axial direction of the heat transfer tube.
FIG. 3 is a view explaining a relationship between the roll angle of the roll straightening setting conditions and a corresponding travel distance of the tube to be corrected. Assuming that d (mm) is an outside diameter of the tube to be corrected 1 and θ (°) is an angle (hereinafter referred to as “roll angle”) formed by an axial center of the tube to be corrected 1 and the rotating axis of the straightening roll R, a travel distance (hereinafter referred to as “feed pitch”) M (mm) of the tube to be corrected 1 per one rotation of the straightening roll R is defined by the following equation (2):M=π·d·tan θ  (2)
FIG. 4 is a view explaining the crush amount of the roll straightening setting conditions. As shown in FIG. 4, the tube to be corrected 1b, to which the crush is applied by the roll straightening, is rolled, while being pressed, and deformed into an elliptic shape. A crush amount ε (mm) is indicated by a difference between an outside diameter d of a pre-deformation tube to be corrected 1a and an opposing rolls clearance s of the straightening rolls Ra and Rb, and corresponds to a rolling reduction of the outside diameter of the tube to be corrected 1. The bend correction is performed to the tube to be corrected 1 by repeatedly rolling while pressing the tube to be corrected 1 across the total length. The crush amount ε (mm) is set by raising/lowering the straightening roll Ra.
FIG. 5 is a view explaining the offset amount of the roll straightening setting conditions. An offset amount δ (mm) is indicated by a deflection in crush/roll height between the central pair of straightening rolls Ra2 and Rb2, and the bend correction is performed by imparting a bending stress to the tube to be corrected 1. The crush height is set by raising the straightening roll Rb2, thereby adjusting the offset amount δ (mm).
As described above, in performing the straightening by the roll straightening machine, it is necessary that a certain level of load such as the crush and the offset be applied onto the tube in order to straighten the bends. However, sometimes dimensional fluctuation such as the ovality associated with the load becomes significant.
Specifically, in the conventional process for straightening the heat transfer tube such as the steam generator and the feed water heater, the tube having the excellent pre-straightening dimensional accuracy, for example, the heat transfer tube to which drawing is performed with a high-pressure drawing machine, sometimes increases in ovality after straightening to deteriorate the S/N ratio compared with the cross-sectional shape of the pre-straightening tube due to the straightening performed by the roll straightening machine.
On the other hand, when the bend correction is insufficiently performed in straightening the heat transfer tube, the interference with other component is frequently generated in assembling the heat transfer tube into the heat exchanger, which makes the assembly work difficult. Accordingly, in straightening the heat transfer tube, it is necessary that the dimensional fluctuation associated with the bend correction be suppressed while the tube bend correction accuracy is ensured. Therefore, there have been conventionally proposed various straightening techniques.
In a straightening process disclosed in Japanese Patent Application Publication No. 61-286025, in order to perform the straightening without deteriorating roundness of the inner surface of the tube used in a hydraulic cylinder tube and the like, the offset is imparted to the tube using a cross opposing type roll straightening machine, and the straightening is performed while a predetermined load which does not substantially impart the crush is applied to the tube.
In a straightening process disclosed in Japanese Patent Application Publication No. 2004-330297, in order to suppress roundness deviation in turning inner and outer surfaces of a cut ring used in a bearing race or the like, a residual stress generated in the tube after the straightening is lowered by a multi-roll straightening machine in which the offset amount is set at 12 mm or more and the crush amount is set at 0.6 mm or less, thereby obtaining a seamless steel tube having little dimensional fluctuation during the turning and excellent roundness.
In a process disclosed in Japanese Patent Application Publication No. 60-184424, the roll offset amount and the crush amount are determined from a relationship between an index indicating a plastic region of the tube and a presumptive offset and crush amounts, picked in advance, and the roll position is set to perform the tube straightening, thereby improving the tube bend and/or roundness.
However, in the straightening processes proposed in Japanese Patent Application Publication Nos. 61-286025, 2004-330297, and 60-184424, it is not intended that the ovality or bends in an axial direction of the tube be corrected in order to enhance the S/N ratio in the eddy current test from the tube inside.
In a heat transfer tube production method disclosed in Japanese Patent Application Publication No. 2000-317521, by using a straightening roll in which at least an outer layer portion of a roll main body is made of elastic material having hardness Hs of 50 to 100 in a spring hardness test (A type) defined by JIS K 6301, the test can be made with a high S/N ratio in the eddy current test from the tube inside.
Although the heat transfer tube obtained by the production method of Japanese Patent Application Publication No. 2000-317521 has the S/N ratio higher than that of the conventional technique, the inspection efficiency does not reach a level satisfying manufacturing-related personnel, and there is still large room to be improved. That is, in order to enhance the inspection efficiency, there is the need to improve the dimensional accuracy of the post-straightening heat transfer tube to enable the test with the higher S/N ratio.